1. Field of the Invention
The present invention relates to an optical pick-up used for recording/reproducing or erasing information on an optical disk, an optical disk apparatus, and an information processing apparatus using the same.
2. Description of the Prior Art
Optical memory technology that uses optical disks having a pit pattern as high-density, large-capacity information storage media has been expanding its application from digital audio disks to video disks, document file disks, and further to data files, etc. In recent years, a high-density optical disk such as DVD-ROM etc. using a visible red laser of wavelength of 630 nm to 670 nm as a light source has become prevalent. Furthermore, an optical disk (DVD-RAM) capable of high density recording has been commercialized. A large capacity of digital data has been able to be recorded on an optical disk easily. Furthermore, CD-R that is highly compatible with CD, which has been used broadly, has been prevalent.
From the above mentioned background, in the information reproducing apparatus using DVD, in addition to DVD-ROM and CD, the reproduction on DVD-RAM and CD-R is important. In the information recording and reproducing apparatus using DVD, in addition to the recording and reproducing function on DVD-RAM, the reproduction on DVD-ROM, CD and CD-R is important.
Since recording/reproducing information on CD-R is carried out by the use of the change in the reflectance of light colors and is optimized to a wavelength of about 800 nm, in other wavelengths of light such as visible light, signals may not be reproduced. Therefore, in order to reproduce information on CD-R, it is desirable that an infrared light source having a wavelength of about 800 nm is used. The optical pick-up provided with a red semiconductor laser for DVD and an infrared semiconductor laser for CD and CD-R has been developed. For simplifying the optical system so as to realize miniaturization and low cost, it is proposed that the above-mentioned two kinds of semiconductor lasers, each having a different wavelength, are integrated into one package.
Referring to FIGS. 20 and 21, an optical pick-up disclosed in JP 10 (1998)-289468 A will be described. FIG. 20 is a schematic view showing a configuration of an optical pick-up 200. In formation recording/reproduction is performed on an optical disk 7. There are a plurality of optical disks having a transparent substrate 220 with different thickness. Recording/reproduction herein denotes recording information on an information recording surface 240 of the optical disk 7 or reproducing information on the information recording surface 240. A conventional optical pick-up apparatus 200 has a first semiconductor laser 100a (wavelength λ=610 nm to 670 nm) as a first light source and a second semiconductor laser 100b (wavelength λ=740 nm to 830 nm) as a second light source. This first semiconductor laser 100a is a light source used for recording/reproducing information on DVD and the semiconductor laser 100b is a light source used for recording/reproducing information on the second optical disk. These semiconductor lasers are used depending upon the kinds of optical disks on which recording/reproducing is performed.
A synthesizer 210 synthesizes a light flux emitted from the first semiconductor laser 100a and a light flux emitted from the second semiconductor laser 100b into one identical optical path (which may be substantially the same optical path) to converge the synthesized light flux onto the optical disk 7 via a converging optical system mentioned below. By using a polarizing prism (a birefringent plate) as the synthesizer 210, the light flux emitted from the first semiconductor laser 100a is allowed to pass through the optical path without changing the optical path as an ordinary ray, and the light flux emitted from the second semiconductor laser 100b is allowed to change the optical path as an abnormal ray. This synthesizer 210 may be a hologram.
The optical converging system including an objective lens 60 and a collimating lens 50 is a means for converging a light flux emitted from the semiconductor laser and forming a light spot on the information recording surface 240 via the transparent substrate 220 of the optical disk 7. An aperture stop 150 limits the light flux to the predetermined number of apertures.
A unit 160 includes a hologram 40 and a photodetector 800, etc. in addition to the first semiconductor laser 100a and the second semiconductor laser 100b, which is shown in detail in FIG. 21. In the unit 160, the first semiconductor laser 100a, the second semiconductor laser 100b and the photodetector 800 are arranged in one plane. A further photodetector 230 is arranged for detecting the light from the semiconductor laser rear part. This photodetector 230 is used for current control of the semiconductor laser with an APC (auto power control) circuit based on the amount of light emitted from a rear part of the semiconductor laser.
Furthermore, in this configuration, a focus error signal is detected by a knife edge method. Therefore, on a photo-receiving surface of the photo detector 800, eight photo receiving elements (photo receiving surfaces), A1-D1, A2-D2 are provided. Furthermore, as a photo diverging means, the hologram 40 is used. This hologram element is divided into four parts such as A to D, and the hologram is arranged so that the light beams passing through the divided surfaces are focused on the photo receiving surface of the photo detecting means 800.
Similarly, for the purpose of achieving the small size optical pick up capable of recording/reproducing information on DVD, CD, CD-R, a configuration in which a photo detector and two semiconductor laser chips each having different wavelength are integrated into one unit is disclosed in besides JP10(1998)-289468 A, JP10(1998)-319318A, JP 10(1998)-21577 A, JP 10 (1998)-64107A, JP 10(1998)-321961A, JP 10(1998)-134388A, JP10(1998)-149559 A, JP10(1998)-241189A, JP10(1998)-124918A, JP9 (1997)-120568A, JP12(2000)-1417A, etc.
The category of DVD includes DVD-RAM, in addition to DVD-ROM. Therefore, it is desirable that recording or reproducing apparatus by the use of DVD can reproduce information on DVD-ROM, DVD-RAM, CD-ROM, and CD-R (CD-RECORDABLE), the latter two of which have been prevalent. Each of these disks has respective standardizations, and the standardization defines respective tracking error (TE) signal detection methods capable of reproducing information stably.
A TE signal of the DVD-ROM can be obtained by the phase difference detection method. The phase difference detection method also is referred to as a differential phase detection (DPD) method. By using the change in the strength of far field pattern (FFP) returning from the optical disk by reflection/diffraction, the TE signal can be obtained with one beam. The method uses a change of the diffracted light by the two-dimensional arrangement of pits. The change of the distribution of the light amount in the diffraction by pit rows is detected by the 4-divided photodetector to compare the phases, thereby obtaining the TE signal. This method is suitable for a reproduction only disk having pit rows.
A TE signal of the DVD-RAM can be obtained by a push-pull (PP) method. The PP method is used mainly for a rewritable optical disk and a write once type optical disk. When the guide groove of the optical disk recording surface of the optical disk is irradiated with a converged light spot, the reflected light accompanies a diffracted light in the direction in which the guide groove extends and the direction perpendicular to the guide groove. The FFP returning to surface of the objective lens has an optical intensity distribution due to the interference of the ±first order diffracted light and zero order diffracted light in the guide groove. Depending upon the positional relationship between the guide grooves and the converging spot, one part of the FFP becomes bright and another part of the FFP becomes dark, or on the contrary, one part of the FFP becomes dark and another part of the FFP becomes bright. TE signals can be obtained by the PP method by detecting the change in the optical intensity by using the 2-divided photodetector.
In both the CD-ROM (which includes CD for audio) and CD-R, TE signals can be obtained by the PP method from the viewpoint of standard. However, as compared with DVD-RAM, the strength of TE signals is weak. Furthermore, the PP method has a problem in that a TE signal offset occurs due to the lens shift. In DVD-RAM, in order to avoid such a problem, an offset compensation zone for TE signals is provided on a part of the information recording surface. However, there is no means for solving the problem of offset in the case of CD-ROM or CD-R. Therefore, as the TE signal detection method, usually a 3-beam method is used in CD-ROM or CD-R.
In the 3-beam method, the grating is inserted into the outward path from a light source to an optical disk and zero order diffracted beam (main beam) and ±first diffracted light beams (sub-beams) of the grating are formed on the optical disk. When the main beam is deviated from the center of the track, one of the sub-beams approaches to the center of the track and the other sub-beam is distant from the center of the track, thus causing the difference in the amount of reflected return light. By detecting this difference, TE signals can be obtained.
As mentioned above, for recording or reproducing information on DVD-ROM, DVD-RAM, and CD-ROM, CD-R, it is necessary to carry out three kinds of methods, i.e., the phase difference method, PP method, 3-beam method. However, in conventional methods, there is no specific example of the configuration capable of corresponding to three types of TE signals detection methods, i.e. the phase difference method, PP method, and 3-beam method.
Furthermore, DVD and CD have different thickness of transparent substrates covering the information recording surface. The standard substrate thickness of DVD is 0.6 mm and the standard substrate thickness of CD is 1.2 mm. By converging light by the use of the common optical converging system common in the optical disk having substrates each having different thickness, a spherical aberration that is an aberration symmetrical with respect to the optical axis, occurs. A large number of methods for recording/reproducing information on DVD and CD by the use of a common light converging system are proposed. Furthermore, DVD has a higher recording density than CD, and even if the red laser light source with a short wavelength is used, the necessary numeral aperture (NA) of the lens is 0.6, which is larger than the NA for CD (0.45). A conventional method such as JP10 (1998)-289468 A discloses a configuration in which the NA is reduced as compared with the NA when reproducing information on CD by using the aperture stop 150.
As mentioned above, information reproduction on CD and DVD are carried out under the remarkably different optical conditions of thickness of substrate, wavelength of light source and NA. Therefore, like in a conventional method, when reproducing information on CD and DVD, in a configuration in which FE signals are detected from the common photo-receiving dividing regions, due to the difference in the optical properties such as the above-mentioned items, the deterioration of properties, for example, FE signal offset, deterioration of FE signal amplitude (signal strength) and the like, occur. Furthermore, in the configuration in which, as shown in FIG. 22 of JP9 (1997)-120568 A (FIG. 5) of JP9 (1997)-120568 A), diffracted lights entering in the different positions on the photodetector due to the difference in the wavelength are received by a continuous photodetector region (for example, 800D), the area of each photodetector region becomes larger, and thus the electric capacitance of the photodetector region is increased, signals of high frequency cannot be detected, and signals cannot be reproduced at high speed.
Furthermore, a configuration suitable for obtaining the excellent signal when reproducing information on DVD and CD, considering a difference in the wavelength and the position of the light emitting spots has not conventionally been considered.